This invention generally relates to improved systems for safe/arm devices, and more particularly to systems for explosive disarming and arming of oil well perforating instruments.
It is common practice in the completion of oil and gas wells to perforate the well casing and the surrounding formations to bring a well into production by the utilization of detonating explosives of high velocity and of the general character and forms known as "shaped charges", as for example, the type described in U.S. Pat. No. 4,387,773, issued to W. A. McPhee and assigned to the assignee of the present invention.
In the operation of a shaped charge perforating gun, a gun assembled body containing a plurality of shaped charges is lowered into a wellbore. The gun is positioned opposite the subsurface formations to be perforated. Electrical signals are passed typically from a surface location through a wireline to one or more blasting caps located in the gun body thereby causing detonation of the blasting caps. The exploding blasting caps in turn transfer a detonating wave to a detonator cord which further causes the shaped charges to detonate. The detonated shaped charges form a energetic stream of high pressure gasses and high velocity particles, a so called "jet", which perforates the well casing and the adjacent formations.
Due to the explosive and dangerous nature of shaped charge perforating guns, when in storage or shipment the primary explosives, commonly blasting caps, must be segregated from the detonator cord and thus the shaped charges. In the past only by physical segregation could one be assured that the perforating guns would not accidentally fire due to spurious ignition of the blasting cap caused by physical shock or static electricity. Unfortunately such physical segregation results in the necessity of assembling the perforating gun at the wellsite. Due to the adverse conditions of the oil field, the technical complexities involved and the time requirements of such an operation, well site assembly is impractical and dangerous. Several safe/arm devices have found use in shaped charge perforating guns. For example, U.S. Pat. No. 4,314,614, describes a system where an interrupter member is inserted within an air gap between an ignitor section and an explosive charge section of a blasting cap. Although such device allows transportation of a loaded, but safed, perforating gun it requires a special electric blasting cap and cannot easily be re-safed once armed. Another such system is described in U.S. Pat. No. 4,011,815. In this system an explosive booster is releasably retained in a first safe position while being selectively movable along the longitudinal dimension of the gun body to an armed position. It should be recognized that such a system requires extensive and costly modifications to the typical perforating gun and adds additional length to the gun member.